Field of the Invention
The present invention relates to a honeycomb filter and a production method for a honeycomb filter.
Discussion of the Background
Particulate matter (hereinafter, also referred to as “PM”) such as soot is contained in exhaust gases discharged from internal combustion engines such as diesel engines, and has raised problems as contaminants harmful to the environment and the human body. Also, people have been worried about influence of toxic gas components such as CO, HC, and NOx contained in exhaust gases on the environment and the human body as well.
For the above reasons, exhaust gas purifying apparatuses for collecting PM or converting toxic gas component in exhaust gases have been used.
Honeycomb structures made of ceramics or the like are used for manufacturing the exhaust gas purifying apparatuses. When exhaust gases are passed through the honeycomb structure, those gases can be converted.
A honeycomb structure for collecting PM in exhaust gases in an exhaust gas purifying apparatus has a large number of cells each sealed at either end thereof and placed longitudinally in parallel with one another with a cell wall interposed therebetween. Therefore, exhaust gases flowing into one of the cells surely pass through the cell wall separating the cells and then flow out from other cells. Therefore, when a honeycomb structure of this kind is installed in an exhaust gas purifying apparatus, PM contained in exhaust gases are collected by the cell walls upon passing through the honeycomb structure. The cell walls of the honeycomb structure function as filters through which the exhaust gases are purified.
In an initial stage of collecting PM in the honeycomb structure, PM enters into a pore of the cell wall, and is collected in the cell wall to clog the pore of the cell wall, so that a so-called “depth filtration” state is generated. In the depth filtration state, PM is deposited in (the pore of) the cell wall. For this reason, immediately after start of PM collecting, there is a problem that the substantial porosity of the cell wall decreases, and a pressure loss rapidly increases.
On the other hand, as a honeycomb structure to be used for converting NOx in exhaust gases in an exhaust gas purifying apparatus, a honeycomb structure for converting NOx in which no end of the cells is sealed and a catalyst for converting NOx is supported on cell walls has been used.
A urea SCR (Selective Catalytic Reduction) device has recently been proposed for converting NOx in exhaust gases.
In the urea SCR device, an aqueous urea solution is sprayed in an exhaust gas purifying apparatus provided with a honeycomb structure in which an SCR catalyst such as zeolite is supported on cell walls. Ammonia is generated due to thermal decomposition of the urea, and the action of zeolite reduces NOx.
Accordingly, a urea SCR device can convert NOx.
Conventionally, a honeycomb structure for collecting PM in exhaust gases and a honeycomb structure for converting NOx are different members, and respectively disposed in different metal casings, and thus they have a large volume in an exhaust gas line. For this reason, decrease of the volume occupied by the exhaust gas purifying apparatus has been desired.
Under such circumstances, a honeycomb filter allowed to support an SCR catalyst such as zeolite on the honeycomb structure for collecting PM in exhaust gases, that is, the honeycomb structure in which each of the cells is sealed at either end thereof has been developed. The honeycomb filter can collect PM in exhaust gases as well as convert NOx.
International Publication WO 2011/042992 discloses a technique of increasing the NOx conversion rate, when the honeycomb filter is applied in the urea SCR device, by controlling the porosity of cell walls of the honeycomb structure.